Positron emission tomography (PET) is a widely established functional imaging method. In the course of an examination, a short-lived radioactive substance is administered to an examination object or a human examination subject and its distribution in the organism is made visible by way of PET. This enables biochemical and physiological functions of the organism to be imaged. Examples of radiopharmaceuticals used for this are molecules that have been marked with a radionuclide tracer which emits positrons.
The high-energy photons resulting during the annihilation of the positron with an electron in the body of the examination subject and emitted at an angle of 180° to one another are registered by way of a plurality of detectors arranged in an annular array around the examination subject. Owing to certain physical processes e.g. preferably photons having an original energy of 511×103 electron-volts can be used for PET. Only coincident events which are recorded by way of two oppositely disposed detectors are tracked. These events can be assigned to the photons emitted at an angle of 180° to one another.
The spatial distribution of the radiopharmaceutical in the examination region is derived from the registered, coincident decay events and a series of sectional images (slices) is computed. A spatial resolution of PET data is typically lower than the spatial resolution of other imaging methods such as e.g. computed tomography (CT) or magnetic resonance tomography (MRT).
As they pass through matter the photons produced during the annihilation are absorbed, the absorption probability being dependent on the path length through the matter and the corresponding absorption parameter μ. The absorption in the tissue is described by way of an attenuation correction factor ACF. The attenuation correction factor ACF is given byACF=EXP(−INT(μ(r)dr)),where EXP designates the exponential function and INT dr denotes a line integral over the path r traveled by the photon from the origin to the detector. To express it in another way, the absorption parameter μ is therefore a measure for the probability of an absorption of a photon within a volume element. The absorption probability can be quantitatively calculated from the value of the absorption parameter μ.
For example, when a quantitative analysis of the PET data is to be carried out in order, say, to obtain a quantification of accumulations of the marked substance in regions of the examination subject, or when PET imaging at a particularly high resolution is to be realized, it may be worthwhile subjecting the PET data to an attenuation correction using the attenuation correction factor ACF. If the attenuation correction is carried out, uncertainties in the determination can have a great influence on the accuracy of the attenuation-corrected PET data. If e.g. the absorption parameter μ is determined only with a specific uncertainty, i.e. there is a significant error in the value of the absorption parameter μ, then the attenuation correction factor ACF may also exhibit an uncertainty due to the fact that the attenuation correction factor ACF is exponentially dependent on the absorption parameter μ.
In order to achieve a maximally accurate correction of the PET data which also takes higher-order, e.g. second-order, effects into account (such as, for instance, PET scattering, referred to as “scatter correction”), a distortion-free attenuation correction map (so-called “μ map”), i.e. a parameter map of the value of the absorption parameter μ, may be required. In addition, so-called “scatter scaling” may necessitate a precise specification of the outline of the object. This is because the correction of the attenuation of the radiation by way of a parameter map of the value of the absorption parameter μ requires knowledge of the position of the attenuating structures and objects.
Various methods of generating such a parameter map are known. It is possible for example to determine the parameter map by way of a combined PET/CT system or a combined PET/MRT system. In the case of a PET/MRT system, a PET system and a magnetic resonance (MR) system may be present integrated in one appliance. For example, it is possible in the case of a determination of the parameter map on the basis of MR data to differentiate experimentally by way of suitable MR recording techniques between e.g. fat, water, lung and background and to assign different values of the absorption parameter μ to the different regions. Corresponding methods are known for CT data.
However, the field of view or measurable volume of MR data is restricted in all three spatial directions due to physical and technical limitations of the magnetic field homogeneity and the linearity of gradient fields. Typically, the basic magnetic field of an MR system is generated by way of a superconducting tube-shaped coil magnet. The patient or examination object is located in the tube inside the magnet. Strong spatial distortions in the MR data occur close to the edge of the tube, i.e. outside the field of view of the MR system. Particularly strong distortions cannot fulfill e.g. high specification requirements in terms of the location fidelity of an MR image or can do so only to a limited extent. The field of view of the MR system is typically specified based on such requirements.
Imaging that is true to the original outside the field of view then cannot be achieved, or can be achieved only to a limited extent, using conventional MR recording techniques. However, since objects that are important for the determination of the parameter map of the following PET measurement, e.g. the arms of a patient, can also be located at these regions outside of the examination object, it may be necessary to determine the parameter map there also. See in this regard: G. Delso et al. “The effect of limited MR field of view in MR/PET attenuation correction” in Med. Phys. 37 (2010) 2804-2812.
It is possible for example to simulate those sections of the parameter map that lie outside the normal field of view of the MR system subsequently from the PET data itself. See in this regard: J. Nuyts et al. “Completion of a Truncated Attenuation Image from the Attenuated PET Emission Data” in IEEE Nucl. Sci. Symp. Conf. Record 2010. However, such a method is generally mathematically complex and time-intensive and requires high computing capacities.
Moreover, such techniques can be subject to restrictions in respect of the PET radiopharmaceuticals which can be used, since many substances, such as rubidium for instance, accumulate only to a limited extent in peripheral regions of an examination subject, e.g. the arms. There may also be restrictions in terms of the resolvable time dynamics, since the accumulation of the radiopharmaceutical itself can follow complicated dynamics. Since in addition the PET data to be corrected itself is drawn upon as a basis for calculating the correction parameter, systematic errors can occur or intrinsic uncertainties can be present.